German daily Frankfurter Allgemeine Zeitung (FAZ) reported that tax authorities were investigating Swiss bank Julius Bär as well as the banking giant UBS, suspected of transferring German clients' funds to Asia, including Singapore.
Widmer-Schlumpf for her part told Swiss newspapers there was no evidence of this practice, adding, "The banks have pledged not to make this sort of transfer for tax reasons."
She noted that a Swiss-German agreement yet to be ratified provided for German assets in Swiss banks to be taxed in Switzerland.
FAZ quoted western North Rhine-Westphalia (NRW) state tax official Manfred Lehmann as saying that illegal movements of cash to Asia by a "hard core" of tax evaders had been observed for some time.
State tax authorities allege Swiss banks are aiding German clients to evade tax by routing funds through their branches in such centres as Hong Kong, China and Singapore.
The daily said that Julius Bär had warned customers that they might figure on a compact disc of data on German clients of Swiss banks suspected of tax evasion recently bought by NRW authorities.
Quoting legal sources, it said some of them had already been consulting lawyers on the issue.
The purchase of bank data by the NRW government has angered the German federal and Swiss authorities, which want to ban the practice under the new treaty.
But NRW finance minister Norbert Walter-Borjans said Friday that it was "an important part" of the tax fraud investigations by Germany's most populous region.
He said NRW had so far spent a "one-digit amount in the millions" on buying the data, which had reaped "approximately 300 million" euros of evaded taxes.
Germany and Switzerland became embroiled in a major row in 2010 when German authorities raided branches of Credit Suisse bank in 13 German cities after buying data on suspected tax dodgers.
Switzerland reacted angrily, saying the data -- bought for a reported €2.5 million -- was stolen in violation of its banking secrecy laws.
The tax treaty still has to be ratified by the upper house of the German parliament, where majority opposition social democrats and Greens consider it to be too soft on evaders.